注：这篇文章参考了https://www.cnblogs.com/snowater/p/8303698.html，而后自己结合hotspot源码看的

1.sun.misc.Unsafe中CAS方法

在sun.misc.Unsafe中CAS方法如下：

1. compareAndSwapObject(java.lang.Object arg0, long arg1, java.lang.Object arg2, java.lang.Object arg3);
2. compareAndSwapInt(java.lang.Object arg0, long arg1, int arg2, int arg3);
3. compareAndSwapLong(java.lang.Object arg0, long arg1, long arg2, long arg3);

来看下openJDK8的hotspot中，unsafe是如何实现的，链接http://hg.openjdk.java.net/jdk8u/jdk8u60/hotspot/file/37240c1019fd/src/share/vm/prims/unsafe.cpp

图1

List-1

UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapObject(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jobject e_h, jobject x_h))UnsafeWrapper("Unsafe_CompareAndSwapObject");oop x = JNIHandles::resolve(x_h);//更新后的值oop e = JNIHandles::resolve(e_h);//期望值oop p = JNIHandles::resolve(obj);//更新的对象HeapWord* addr = (HeapWord *)index_oop_from_field_offset_long(p, offset);//获取偏移地址，可以理解为获取要更新的属性的内存地址oop res = oopDesc::atomic_compare_exchange_oop(x, addr, e, true);//调用方法执行CAS操作jboolean success  = (res == e);//atomic_compare_exchange_oop的返回值是否等于期望值，如果等于，则success为trueif (success)update_barrier_set((void*)addr, x);//更新memory barrierreturn success;
UNSAFE_END

atomic_exchange_oop声明在http://hg.openjdk.java.net/jdk8u/jdk8u60/hotspot/file/37240c1019fd/src/share/vm/oops/oop.hpp中，如下

List-2

  static oop atomic_compare_exchange_oop(oop exchange_value,volatile HeapWord *dest,oop compare_value,bool prebarrier = false);

实现在http://hg.openjdk.java.net/jdk8u/jdk8u60/hotspot/file/37240c1019fd/src/share/vm/oops/oop.inline.hpp中，如下List-3所示：

List-3

inline oop oopDesc::atomic_compare_exchange_oop(oop exchange_value,volatile HeapWord *dest,oop compare_value,bool prebarrier) {if (UseCompressedOops) {if (prebarrier) {update_barrier_set_pre((narrowOop*)dest, exchange_value);}// encode exchange and compare value from oop to TnarrowOop val = encode_heap_oop(exchange_value);narrowOop cmp = encode_heap_oop(compare_value);narrowOop old = (narrowOop) Atomic::cmpxchg(val, (narrowOop*)dest, cmp);//核心// decode old from T to oopreturn decode_heap_oop(old);} else {if (prebarrier) {update_barrier_set_pre((oop*)dest, exchange_value);}return (oop)Atomic::cmpxchg_ptr(exchange_value, (oop*)dest, compare_value);//核心}
}

如上的List-3所示，核心的CAS调用Atomic::cmpxchg(val, (narrowOop*)dest, cmp)和Atomic::cmpxchg_ptr(exchange_value, (oop*)dest, compare_value)。

cmpxchg的实现是在http://hg.openjdk.java.net/jdk8u/jdk8u60/hotspot/file/37240c1019fd/src/share/vm/runtime/atomic.cpp中，如下List-4所示

List-4

jbyte Atomic::cmpxchg(jbyte exchange_value, volatile jbyte* dest, jbyte compare_value) {assert(sizeof(jbyte) == 1, "assumption.");uintptr_t dest_addr = (uintptr_t)dest;uintptr_t offset = dest_addr % sizeof(jint);volatile jint* dest_int = (volatile jint*)(dest_addr - offset);jint cur = *dest_int;//对象当前值jbyte* cur_as_bytes = (jbyte*)(&cur);jint new_val = cur;jbyte* new_val_as_bytes = (jbyte*)(&new_val);new_val_as_bytes[offset] = exchange_value;//这里有个for循环，资料上说“ 比较当前值与期望值，如果相同则更新，不同则直接返回”？while (cur_as_bytes[offset] == compare_value) {jint res = cmpxchg(new_val, dest_int, cur);if (res == cur) break;cur = res;new_val = cur;new_val_as_bytes[offset] = exchange_value;}return cur_as_bytes[offset];
}

cmpxchg_ptr的实现由不同的系统而实现不同，以64位linux为例，是在http://hg.openjdk.java.net/jdk8u/jdk8u60/hotspot/file/37240c1019fd/src/os_cpu/linux_x86/vm/atomic_linux_x86.inline.hpp中